In the construction of electronic control devices it is common practice to use a printed wiring board which has an insulated surface that in turn supports a plurality of conductive surfaces as one element of the device's structure. The printed wiring board supports a number of electronic components that are interconnected by the printed wiring board's conductive areas. This type of structure, if exposed to a moist atmosphere, can fail due to condensation shorting out electrical paths between the conductive areas. This in turn causes paths to develop across the printed wiring board that allows the migration of potentials from one type of voltage source to another. In electronic control devices that have no safety function, this is an inconvenience, but not a safety problem. In devices that have safety functions, the shorting out of the conductive areas by moisture, and the subsequent migration of unwanted potentials, can create an unsafe mode of operation.
A typical type of control device that provides a safety function is a burner control apparatus in which the device is responsible for the safe ignition of the fuel, with subsequent monitoring of a flame. Typical of this type of device is an S89C Hot Surface Ignition Control as manufactured by Honeywell Inc. The S89C utilizes a hot surface ignitor which, when energized, ignites a gaseous fuel from a burner. The hot surface ignitor then acts as a flame rod to provide a flame rectification signal that monitors the presence of a flame at the burner. Typically, the flame rectification signal is a very small rectification signal, and is usually negative in potential with respect to a ground circuit or a positive potential used in the balance of the device. These types of units, if exposed to moisture, can have condensation of their printed wiring boards which causes migration of voltages that can cause the flame sensing circuit to believe a flame exists when no flame exists. This type of problem has been encountered in installations in food processing locations. It is quite common in these environments for the control device to be exposed to atmospheric moisture or to the spray of water from clean-up activities.
This type of problem can be readily solved by providing completely sealed electronic control devices, or by "potting" the device. Potting is a term used to generically refer to electronic equipment which has been filled with a plastic-like material that either becomes hard or semi-hard, but totally seals the device from moisture. The use of a totally water-tight enclosure or potting creates both a cost penalty, and a situation that makes repair of the device difficult or impossible.